IP Multimedia services provide a dynamic combination of voice, video, messaging, data, etc. within the same session. By growing the number of basic applications and the media which it is possible to combine, the number of services offered to the end subscribers will grow, and the inter-personal communication experience will be enriched. This will lead to a new generation of personalised, rich multimedia communication services, including so-called “combinational IP Multimedia” services.
IP Multimedia Subsystem (IMS) is the technology defined by the Third Generation Partnership Project (3GPP) and ETSI TISPAN group to provide IP Multimedia services over mobile communication networks. IMS provides key features to enrich the end-subscriber person-to-person communication experience through the use of standardised IMS Service Enablers, which facilitate new rich person-to-person (client-to-client) communication services as well as person-to-content (client-to-server) services over IP-based networks. The IMS makes use of the Session Initiation Protocol (SIP) to set up and control calls or sessions between subscriber terminals (or subscriber terminals and application servers). The Session Description Protocol (SDP), carried by SIP signalling, is used to describe and negotiate the media components of the session. Whilst SIP was created as a subscriber-to-subscriber protocol, IMS allows operators and service providers to control subscriber access to services and to charge subscribers accordingly.
By way of example, FIG. 1a illustrates schematically a telecommunications network showing an example of the IMS network architecture used for roaming users when they place a call in a Visited Public Land Mobile Network (V-PLMN). In this example, a calling party A is visiting the V-PLMN for party A (V-PLMN A or Visited operator for party A) places a call to a called party B visiting V-PLMN for party B (V-PLMN B). Each PLMN includes some IMS architecture such as Interconnection Border Control Function (IBCF) or Translation Gateway nodes (TrGw) that connect the PLMN with each other via core networks for example those based on Internetwork Packet Exchange (IPX) 1, 2, 3, and 4. An IBCF is an interconnecting gateway between an IMS network 102 and other IP networks e.g. another IMS networks or core networks etc. Each PLMN includes some IMS architecture such as registrar nodes associated with the various access domains (or networks) such as Call/Session Control Functions (CSCFs), which operate as SIP proxies within the IMS. The 3GPP architecture defines three types of CSCFs: the Proxy CSCF (P-CSCF) which is the first point of contact within the IMS for a SIP terminal i.e. calling party A; the Serving CSCF (S-CSCF) (not shown) which provides services to the subscriber that the subscriber is subscribed to; and the Interrogating CSCF (I-CSCF) whose role is to identify the correct S-CSCF and to forward to that S-CSCF a request received from a SIP terminal via a P-CSCF.
Within the IMS network architecture, Application Servers (ASs) are provided for implementing IMS service functionality. ASs provide services to end users in an IMS system, and may be connected either as end-points over the 3GPP defined Mr interface, or “linked in” by an S-CSCF over the 3GPP defined ISC interface. In the latter case, Initial Filter Criteria (IFC) are used by an S-CSCF to determine which ASs should be “linked in” during a SIP Session establishment (or indeed for the purpose of any SIP method, session or non-session related). The IFCs are received by the S-CSCF from a home subscriber server (HSS) during the IMS registration procedure as part of a user's or subscriber's Subscriber Profile.
The user equipment (UE) for a calling or called party may comprise or represent any device used for communications. Examples of user equipment that may be used in certain embodiments of the described network are wireless devices such as mobile phones, terminals, smart phones, portable computing devices such as lap tops, handheld devices, tablets, netbooks, computers, personal digital assistants and other wireless communication devices, or wired communication devices such as telephones, computing devices such as desktop computers, set-top boxes, and other fixed communication devices.
When the calling party A (UE A) places a call to the called party B (UE B) the call set-up process involves an originating call associated with UE A and a terminating call associated with UE B. The terms “originating call” and “terminating call” may comprise or represent the connection set-up signalling in relation to UE A or UE B, respectively. Examples of originating or terminating calls that may be used in certain embodiments of the described network include, but are not limited to, the connection set-up signalling enabling a communication connection to be made between UE A and UE B in the two call halves model. The originating call is the connection set-up signalling for UE A in the first call half and the terminating call is the connection set-up signalling for connecting the call with UE B in the second call half.
When the calling party A originates a call, the basic procedure involves the P-CSCF of the V-PLMN A receiving the originating call and routing the call from V-PLMN A to the Home PLMN for party A (H-PLMN A or Home operator for A party) via the corresponding IBCF/TrGWs. Generally, the H-PLMN or home network identifies the PLMN or network in which the profile of a user such as calling party A or called party B (e.g. a subscribers profile) is held. The H-PLMN A routes the originating call to the H-PLMN B based on the call number (B number) indicated in the originating call. Since the call number relates to a called party B in another network, the H-PLMN A routes the originating call to H-PLMN B (Home operator of B party) and it is then routed to V-PLMN B (Visited operator of B party), where the C-CSCF of V-PLMN B forwards the call to the called party B. The media is “anchored” in every network that the originating call traverses, i.e. follows the path of the call i.e. a call chain.
During the call routing procedure, the H-PLMN A may perform so-called number normalisation. In non-roaming cases, when calling party A places a call based on the H-PLMN A numbering plan using short numbers (e.g. 08-71912345 instead +46871912345), then H-PLMN A performs the necessary number conversion in-order to correct the number format according its own numbering plan. In another example, when calling party A is not roaming and dials a service number (e.g. 118118), as this number is recognised as a home service number, then the call is routed to the relevant HPLMN A service or servicer operator.
According to 3GPP Technical Standard 23.228 “IMS” Stage 2 Release 11 and other 3GPP Standards, even if the calling party A is roaming, the H-PLMN A IMS network will be within the call chain for all calls. This means that if calling party A is an IMS (e.g. VoLTE) roamer in VPLMN-A, then when calling party A dials a short number according to the dialing plan of VPLMN-A. For example, such calls may include when calling party A tries to call a called party B within the same visited network using a local number format (e.g. an inbound roamer from Germany in Sweden, dials 08-719 12345 to reach somebody in Sweden). Alternatively, calling party A (an inbound roamer) may dial a service number in the V-HPLMN A (e.g. #TAXI to get to closest taxi company in the visited network).
However, based on current IMS routing procedures, the originating call of calling party A will be routed to HPLMN-A and particularly to an IMS Application Server and multimedia telephony service (MMTel) AS (not shown) within H-PLMN A. However, since the intention of the calling party A is unknown to H-PLMN A, then an incorrect routing of the call may result due to overlapping service numbers (e.g. same service numbers are used for different purposes in H-PLMN A and V-PLMN A). Alternatively, the call may simply fail due to the number being based on a local numbering plan in the V-PLMN A, which may be unknown to the H-PLMN A network.
Therefore, there is a significant need to optimise handling of originating calls in a telecommunications network including an IMS network to minimise or even eliminate incorrect or failed routing of originating calls from inbound roaming users.